The present invention generally relates to the art of plasma processing and more particularly to a plasma processing method and apparatus for eliminating damage in the plasma processing of a substrate.
The art of plasma processing, including plasma-etching and plasma CVD, is used extensively in the fabrication of various semiconductor devices. Further, plasma processing is also used to produce flat panel display devices such as a liquid crystal display device or a plasma display device.
FIG. 1 shows the construction of a typical conventional plasma-etching apparatus 100 used for etching an insulating film.
Referring to FIG. 1, the parallel-plate plasma-etching apparatus 100 includes a processing chamber 101 in which a lower electrode 102 and an upper electrode 103 are accommodated in parallel relationship. The lower electrode 102 functions as a susceptor and supports thereon a substrate W, while the upper electrode 103 is provided so as to face the lower electrode 102.
The processing chamber 101 is supplied with an etching gas such as a mixture of C4F8, Ar and O2 and plasma is formed in the processing chamber 101 by supplying high-frequency power of 60 Mhz from a high-frequency source 104 to the upper electrode 103 via an impedance-matching device 105. When a plasma-etching process is carried out on an insulation film formed on the substrate W, a low-frequency bias of 2 MHz is supplied further to the lower electrode 102 from a low-frequency power source 108 via an impedance-matching device 109.
When an a.c. power of low frequency is used for the low-frequency bias, the firing voltage of discharge, above which voltage an electric discharge starts in the processing chamber 101, increases substantially, provided that the pressure inside the processing chamber 101 is held low. Thus, no firing of plasma occurs.
In the case of applying a plasma-etching process to an insulation film formed on the substrate W by using the parallel-plate plasma-etching apparatus 100, it has been practiced to activate the high-frequency power source 104 to start a plasma in the processing chamber 101, and the activation of the low-frequency power source 108 is started thereafter to supply the low-frequency bias to the lower electrode 102. By doing so, it is possible to avoid the problem of sudden impedance change caused in the lower electrode 102 with the firing of the plasma and the associated problem of sudden change of load of the low-frequency power source 108.
Meanwhile, Applicants have discovered, when the conventional parallel-plate plasma etching apparatus such as the apparatus 100 of FIG. 1 is used in the processing of a substrate of advanced, leading-edge semiconductor devices, such as submicron or sub-quarter-micron devices, that the ultrafine semiconductor structures formed on the substrate tend to be damaged as a result of the plasma processing and that a production yield of the semiconductor device is deteriorated.
FIGS. 2A and 2B show the construction of the test piece used in the foregoing experiment conducted by the Applicants.
Referring to FIG. 2A, a Si wafer corresponding to the substrate W of FIG. 1 carries thereon a number of test elements EL, and each of the test elements EL is constructed on a Si substrate 41 corresponding to the Si wafer W as represented in FIG. 2B.
Referring to FIG. 2B, the Si substrate 41 carries thereon a field oxide film 42 defining an active region, while the active region thus defined is covered with a thermal oxide film 43 having a thickness of typically about 5 nm. Further, an electrode pattern 44 of polysilicon is formed on the thermal oxide film 43.
In the experiments, the test elements EL are formed to have an antenna ratio, which is defined as the ratio of the area of the electrode pattern 44 to the area of the thermal oxide film 43, of 260,000, and a plasma-etching process is conducted while setting the separation between the lower electrode 102 and the upper electrode 103 to 19 mm.
According to the experiment, it was discovered that the proportion of the defective test elements EL on the wafer W reaches as much as 35% and that the breakdown voltage of the thermal oxide film 43 is degraded substantially in such defective test elements. Further, it was recognized that the proportion of such defective devices increases when the plasma etching apparatus of FIG. 1 is used for processing ultrafine semiconductor devices.
FIG. 3 shows the proportion of the defective test elements observed in the case the substrate of FIGS. 2A and 2B is subjected to a plasma etching process in the plasma etching apparatus of FIG. 1, wherein the designation xe2x80x9cCWxe2x80x9d in FIG. 1 indicates a continuous wave, while the designation xe2x80x9c58k,xe2x80x9d xe2x80x9c130kxe2x80x9d and xe2x80x9c260kxe2x80x9d represent the antenna ratio.
Referring to FIG. 3, it can be seen that the proportion of the defective elements changes depending on the size of the gap, or gap distance, between the lower electrode 102 and the upper electrode 103. Further, the proportion of the defective elements changes on the antenna ratio. As long as the gap distance is set to a value used commonly in the plasma etching process, occurrence of substantial defects cannot be avoided. The relationship of FIG. 3 also indicates that the proportion of defects increases with increasing antenna ratio when the gap distance between the electrodes 102 and 103 is held constant.
Accordingly, it is a general object of the present invention to provide a novel and useful plasma processing method and apparatus wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a plasma processing method and apparatus capable of minimizing the proportion of defective devices formed at the time of the plasma processing.
Another object of the present invention is to provide a plasma processing method conducted in a plasma processing apparatus having a processing chamber, an electrode provided in said processing chamber for supporting a substrate thereon, and a plasma generator provided in said processing chamber, said method comprising the steps of:
(A) supplying a first electric power of a first frequency to said electrode such that said first electric power does not start a plasma in said processing chamber; and
(B) supplying a second electric power of a second frequency to said plasma generator such that said second electric power causes said plasma generator to start a plasma in said processing chamber,
wherein said step (A) is conducted such that said first electric power is supplied to said electrode prior to said start of said plasma in said step (B) by said plasma generator.
Another object of the present invention is to provide a plasma processing method conducted in a plasma processing apparatus having a processing chamber, an electrode provided in said processing chamber for supporting a substrate thereon, and a plasma generator provided in said processing chamber, said method comprising the steps of:
(A) supplying an a.c. power to said electrode such that said a.c. power does not start a plasma in said processing chamber; and
(B) supplying a microwave power to said plasma generator such that said microwave power causes said plasma generator to start a plasma in said processing chamber,
wherein said step (A) is conducted such that said a.c. power is supplied to said electrode prior to start said plasma in said step (B) by said plasma generator.
Another object of the present invention is to provide a plasma processing method conducted in a plasma processing apparatus having a processing chamber and an electrode provided in said processing chamber for supporting a substrate thereon, said method comprising the steps of:
(A) supplying a first electric power of a first frequency to said electrode such that said first electric power does not start a plasma in said processing chamber; and
(B) supplying a second electric power of a second frequency to said electrode such that said second electric power causes said electrode to start a plasma in said processing chamber,
wherein said step (A) is conducted such that said first electric power is supplied to said electrode prior to said start of said plasma in said step (B) by said electrode.
Another object of the present invention is to provide a plasma processing apparatus, comprising:
a processing chamber;
a first electrode provided in said processing chamber, said first electrode supporting a substrate thereon in said processing chamber;
a second electrode provided in said processing chamber so as to face said first electrode;
a first power source supplying a first electric power of first frequency to said first electrode such that said first electric power does not cause said first electrode to start a plasma in said processing chamber;
a second power source supplying a second electric power of a second, higher frequency to said second electrode such that said second electric power causes said second electrode to start a plasma in said processing chamber,
said first electrode supplying said first electric power to said first electrode prior to starting of said plasma at said second electrode in response to supplying of said second electric power from said second power source.
Another object of the present invention is to provide a plasma processing apparatus, comprising:
a processing chamber;
an electrode provided in said processing chamber, said electrode supporting thereon a substrate in said processing chamber;
a first power source supplying a first electric power of first frequency to said electrode such that said first electric power does not cause said electrode to start a plasma in said processing chamber;
a second power source supplying a second electric power of a second, higher frequency to said electrode such that said second electric power causes said electrode to start a plasma in said processing chamber,
said first electrode supplying said first electric power to said electrode prior to starting of said plasma at said electrode in response to supplying of said second electric power to said electrode from said second power source.
According to the present invention, it is possible to cover the surface of the electrode carrying the substrate by an ion sheath immediately when a firing of plasma has occurred, by supplying a low-frequency bias, typically 2 MHZ or less in frequency, to the foregoing electrode in advance to the firing of the plasma. The low-frequency bias does not cause firing of plasma, and the ion sheath thus formed effectively protects the electrode and the substrate supported on the substrate from being touched by the plasma. As a result, no charge-up electric current caused by non-uniform charge-up of the substrate flows through the substrate, and the damaging of the semiconductor structures on the substrate is successfully eliminated. As long as the low frequency bias is supplied to the electrode with a magnitude sufficient to induce the ion sheath at the time of firing of the plasma, it is possible to choose the timing of supplying of the second electric power arbitrarily with respect to the timing of supplying of the first electric power. For example, the timing of starting the supply of the second electric power to the plasma generator may be earlier or later than the timing of supplying of the low-frequency bias to the electrode, provided that the supplying of the second electric power causes no firing of the plasma until the first electric power reaches a magnitude sufficient for inducing the desired ion sheath on the surface of the electrode. Alternatively, the supply of the second electric power may be started concurrently to the supply of the first electric power, provided that the supply of the second electric power causes no firing of the plasma until the first electric power reaches a sufficient magnitude for inducing the ion sheath on the surface of the electrode.
Another object of the present invention is to provide a plasma processing method conducted in a plasma processing apparatus having a processing chamber, an electrode provided in said processing chamber for carrying a substrate thereon and a plasma generator provided in said processing chamber, said method comprising the steps of:
(A) supplying a first electric power of a first frequency to said electrode;
(B) supplying a second electric power of a second frequency to said plasma generator such that said second electric power causes said plasma generator to start a plasma;
(C) turning off a supply of said second electric power to said plasma generator; and
(D) turning off a supply of said first electric power to said electrode,
wherein said step (C) is conducted no later than said step (D).
According to the present invention, it is also possible to eliminate the charge-up current to flow through the substrate at the time of turning off the plasma, by conducting the turning-off of the plasma in the state that the electrode is supplied with the low-frequency bias sufficient to induce an ion sheath. By maintaining the ion sheath at the time of extinguishing the plasma, the problem of non-uniform charge-up of the substrate caused by the shrinking plasma is effectively eliminated.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.